1. Field of the Invention
The present invention relates to polyimides as optical polymers for use in manufacturing optical communication devices for optical communications, and to methods of preparing polyimides, and methods of forming multiple polyimide film layers.
2. Description of the Related Art
An ideal optical material for use in manufacturing optical communication devices has low optical loss at a wavelength of 1300-1500 nm for optical communications. Optical loss in this wavelength range is caused by overtones of harmonics due to stretching and deformation vibrations of carbon and hydrogen bonds (C--H) in alkyl, phenyl and other similar functional groups. Thus, using a common polymer as a material for optical waveguides that utilize light in this near infrared wavelength range is not desirable due to a large optical transmission loss. In order to reduce optical transmission loss, the light absorption wavelength of the polymer should be shifted from a near infrared light wavelength region to a longer or shorter wavelength region. To this end, a method for substituting hydrogen of the carbon and hydrogen (C--H) bond by fluorine (F) has been suggested.
Also, an optical material for use in manufacturing an optical communications device such as an opto-electronic integrated circuit (OEIC), an opto-electrical mixed wiring board (OEMWB), a hybrid integration device or a plastic optical fiber requires thermal stability during a manufacturing process. Because the thermal resistance of the optical material is very important, glass transition temperature, thermal decomposition temperature, thermal expansion coefficient and birefringence of the optical material should be carefully considered.
Polyimide is widely known as a polymer having good thermal resistance. Because polyimide has a resistance to heat at a high temperature, for example, approximately 400.degree. C., great efforts are being made to utilize polyimide as a material for optical communications.
However, in general, polyimides have many C--H bonds in the molecules, so that optical absorption loss is large at the near infrared region. To avoid this problem, a polyimide whose hydrogen of C--H bond is partially or completely substituted for fluorine (F) has been reported.
However, as the F content in the polyimide increases, solubility of the polyimide in the organic solvent which is commonly used in the formation of a polyimide film, also increases. Thus, in the case of forming multiple polyimide films, a lower polyimide film is partially solubilized in the organic solvent used to form an upper polyimide film, causing cracks to occur in the lower polyimide film. Such cracks cause optical scattering loss, thereby increasing optical loss. Also, polyimide films has a large birefringence, so that there is a problem in that optical waveguiding characteristics vary in accordance with the degree of polarization during optical waveguiding.